For several decades, U.S. hypertension treatment guidelines have recommended a low-dose diuretic such as hydrochlorothiazide (HCTZ) or chlorthalidone (CTD) as initial monotherapy in a stepped-care approach or as part of an initial two-drug combination. In numerous large clinical trials, CTD-based regimens have significantly reduced rates of cardiovascular events such as stroke, heart failure, and cardiovascular mortality compared with placebo, usual care, or active comparators. In contrast, few outcome studies have compared HCTZ-based regimens with other treatments, and they have generally found HCTZ to be less effective than non-thiazide comparators in preventing cardiovascular events. The two drugs have never been compared directly in a large trial. Despite the empirical evidence favoring CTD, HCTZ is much more widely used in clinical practice. The proposed R34 is a pilot study conducted in two large integrated health systems to test methods and feasibility for an eventual pragmatic randomized trial comparing the effectiveness of HCTZ and CTD for preventing cardiovascular events and mortality. Using a cluster-randomized design, we will allocate 40 primary care physicians and their adult hypertensive patients who currently use HCTZ (N=2,000) to either convert HCTZ users to CTD (intervention group) or to continue HCTZ (usual care group). The pilot study and planned full- scale trial will use existing health care infrastructure and electronic health records to identify eligible study subjects, distribute study medication, and collect operational and outcomes data. For this pilot study, Aim 1 will document that the intervention is delivered as intended. Aim 2 will compare safety in intervention and usual care patients. Aim 3 will refine the pilot study design and identify critical factors for intervention success using a mixed-methods approach. Aim 4 will refine estimates of sample size. Finally, Aim 5 will determine the costs of ascertaining outcomes and estimate per-participant costs for the full-scale trial. The pilot study will produce data that are necessary an sufficient to inform the planning of a full-scale trial comparing CTD and HCTZ and will advance the development of methods for pragmatic trials.